How Is Obsidian Formed?

Obsidian is a type of glass that is formed when very thick volcanic lava cools down very quickly. It is a natural glass – humans are not involved in its production.

Obsidian has some fairly unusual properties; it is very high in silica – the content can range from 35 per cent to as high as 80 per cent and it is very low in water. This makes it harder than the man-made glass we used in windows.

Obsidian is also generally jet black and completely opaque, so would not be useful for seeing much. If the obsidian contains other minerals, such as haematite, it can also take on a red or brown lustre but is always very rich and dark. Tiny bubbles in the lava may form during cooling and these create a sparkle to the glass that has fascinated people for centuries. It is used as a semi-precious stone and is used to make unusual and attractive jewellery.

Obsidian is jet black and is formed by thick lava cooling very quickly

Obsidian is a natural glass formed when lava is cooled quickly and no crystals have formed. It is a very dark black and like Kath18 said, it is not very useful for looking out of.

Don't listen to the person two posts above me, she/he doesn't know what she/he is talking about. It is true that lava is molten rock that breaks through the Earths surface. The problem in her/his entire statement is that obsidian forms from LAVA not MAGMA. As everyone has already answered, lava needs to cool quickly to form obsidian. How could magma possibly cool fast enough inside the earths crust to achieve the formation of obsidian? The only possibility is magma penetrating a cavern of water trapped within the Earths crust. Science tends make definitions according to the majority meaning that obsidian forms from LAVA.

It is made when lava cools quickly and there is absence of crystals

It's MAGMA not lava magma is melted rock under earths crust lava is magma when it breaks the surface.